HETEROGENEOUS CATALYSIS AN INTRODUCTION

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HETEROGENEOUS CATALYSISAN INTRODUCTION

• Paul Ratnasamy
• National Chemical Laboratory
• Pune-411008, India

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Why R D in catalysis is important

• -27 of GNP and 90 of chemical industry
  involve products made using catalysts (food,
  fuels, polymers, textiles, pharma/agrochemicals,et
  c)
• -For discovery/use of alternate sources of
  energy/fuels/ raw material for chem industry.
• -For Pollution control-Global warming.
• - For preparation of new materials (organic
  inorganic-eg Carbon Nanotubes).

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Catalysis is multidisciplinary
(physics,chemistry chem engg)

• The catalyst is an inorganic solidCatalysis is a
  surface phenomenonsolid state and surface
  structures play important roles.
• Adsorption,desorption and reaction are subject to
  thermodynamic, transport and kinetic
  controls(chem engg)
• adsorbate-substrate and adsorbate - adsorbate
  interactions are both electrostatic and
  chemical(physical chemistry).
• The chemical reaction is organic chemistry.

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Green Chemistry is Catalysis

• Pollution control(air and waste streams
  stationary and mobile)
• Clean oxidation/halogenation processes using
  O2,H2O2(C2H4O, C3H6O, ECH)
• Avoiding toxic chemicals in industry
• ( HF,COCl2 etc.)
• Fuel cells( H2 generation)

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Catalysis in Nanotechnology

• Methods of Catalyst preparation are most suited
  for the preparation of nanomaterials .
• Nano dimensions of catalysts.
• Common prep methods.
• Common Characterization tools.
• Catalysis in the preparation of carbon nanotubes.

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Hetrogeneous Catalysis-Milestones in Evolution-1

• 1814- Kirchhoff-starch to sugar by acid.
• 1817-Davy-coal gas(Pt,Pd selective but not
  Cu,Ag,Au,Fe)
• 1820s Faraday H2 O2 ?H2O(Pt)C2H4 and S
• 1836- Berzelius coinsCatalysis
• 1860-Deacons Process 2HCl0.5O2 ? H2O Cl2
• 1875-Messel.SO2 ? SO3 (Pt)
• 1880-Mond.CH4H2O ? CO3H2(Ni)
• 1902-Ostwald-2NH32.5O2 ?2NO3H2O(Pt)
• 1902-Sabatier.C2H4H2 ? C2H6(Ni).
• 1905-Ipatieff.Clays for acid catalysed reactions
  isomerisation, alkylation, polymerisation.

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Milestones in Evolution-2

• 1910-20 NH3 synthesis (Haber,Mittasch)
  Langmuir
• 1920-30-Methanol syn(ZnO-Cr2O3) TaylorBET
• 1930-Lang-Hinsh Eley -Rideal models FTsynEO
• 1930-50Process Engg FCC / alkylatesacid-base
  catalysisReforming and Platforming.
• 1950-70 Role of diffusion Zeolites, Shape
  Selectivity Bifunctional cataoxdn cat-HDS
  Syngas and H2 generation.
• 1970- Surface Science approach to catalysis(Ertl)
• 1990 - Assisted catalyst design using
• -surface chem of metals/oxides, coordination
  chemistry
• - kinetics,catalytic reaction engg
• - novel materials(micro/mesoporous
  materials)

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Catalysis in the Chemical Industry

• Hydrogen Industry(coal,NH3,methanol, FT,
  hydrogenations/HDT,fuel cell).
• Natural gas processing (SR,ATR,WGS,POX)
• Petroleum refining (FCC, HDW,HDT,HCr,REF
• Petrochemicals(monomers,bulk chemicals).
• Fine Chem.(pharma, agrochem, fragrance,
  textile,coating,surfactants,laundry etc)
• Environmental Catalysis(autoexhaust, deNOx, DOC)

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PHYSICAL ADSORPTION

• Steps in a catalytic Reaction
• - Diffusion of reactant (bulk, Film, surface)
• - Adsorption( physical ? chemical)
• -Surface reaction
• - Desorption and diffusion of products
• Physical Adsorption
• - Van der Waals forcesBET surface area
• Pore Size distribution ( Wheeler, de Boer, BJH)
• Influence of pore size on reaction order,
  temperature coefficient, selectivity, Influence
  of poisons

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CHEMISORPTION

• Langmuir isotherm Langmuir Hinshelwood and
  Eley- Rideal mechanisms of surface
  reactionsKinetics of adsorption-Elovich
  equation.
• Uses of chemisorption (1)probes (H2,CO,NH3,
  pyridine,CO2) for fraction of catalytically
  active surface (only 0.1 in cracking)(2)Do
  chemisorbed species actually participate in
  reactions(isotope exchange)(3) changes in
  surface structures on adsorption(S, H2, O2,
  H2O2).

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The Sabatier Principle

• There is an optimum of the rate of a catalytic
  reaction as a function of the heat of
  adsorption- Sabatier,1905 If the adsorption is
  too weak,the catalyst has little effectIf too
  strong, the adsorbates will be unable to desorb
  from the surfaceHence,the interaction between
  reactants or products with surface should be
  neither too strong nor too weak.

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Sabatier Principle -Optimal basicity results in
high carbonate yields (MMM 90(2006)314)
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How catalysts accelerate rates of chemical
reactions

• H20.5O2 ? H2O ? G 0298 -58 Kcal/mol
• In the gas phase
• D(H-H) 103 and D(O-O)117 Kcal/mol
• E 10 Kcal/mol for HO2 or H2O ? HO2 or
  H2O.
• Hence,kinetically gas-phase reaction improbable.
• Pt forms Pt-H and Pt-O bonds with E
  0Moreover,
• Pt-H Pt-O ? Pt-OH ? Pt -OH2 has E 0 .

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Turnover frequencies, Rates and numbers

• CATALYSIS IS A KINETIC PHENOMENON
• Sequence of elementary steps in steady state
  diffusion (bulk,film,surface) -
  adsorption-reaction-desorption-diffusion
• TOF number of product molecules formed per unit
  area per sec(molecules.cm-2.sec-1)
• TOF number of product molecules formed per
  active site per sec(molecules.sec-1) only if
  active site is known.
• TOT 1/TOF turnover time, time necessary to
  form a product molecule(sec)
• TOR Turnover rate TOF X Surface area
• TON TOF X total reaction timeTON1(
  stoichiometry)
• TON must be gt100 to be industrially
  useful.

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Conversions,Rates and Rate constants

• Conversion Reactant converted
• Reaction rate kp X f(Pi) or kc X f(Ci)
• k Aexp(-?E/RT)A is temp independent.
• TOFs between 0.0001 and 100 in industry Temp
  adjusted to get the desired rates.
• ?E 35-45 Kcal/mol for isom,cyclisation,
  cracking,dehydo/hydrogenolysisHighT needed.
• ?E 6-12 Kcal/mol for hydrogenation

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The Compensation Effect

• k A exp(-?E/RT)
• For a given reaction, over different catalysts, A
  increases linearly with ?E so that k remains
  constant
• ln A ? (?E / R? ) ? is a constant and ?
  is the isokinetic temp,when the rates on all
  catalysts are equal A plot of ln A vs ?E gives
  a linear plot with ve slope.

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Compensation effect for the methanation
reactionLogarithm of preexponential factor vs
apparent activation energy
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The Active SiteH.S.Taylor,Proc Roy Soc
(London)A108(1925)105

• There will be all extremes between the case in
  which all the atoms in the surface are active and
  that in which relatively few are so active .
• The amount of surface which is catalytically
  active is determined by the reaction catalyzed.

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Active Sites-MetalsStructure sensitivity of
Catalytic reactions over metals

• Structure Sensitive if rate changes markedly when
  crystallite/particle size is changed active
  site comprises ensemble of many metal
  atomssteps edges. eghydrogenolysis,H2-D2
  exch, steam reform,coking, aromatization etc
• Structure Insensitive if rate is independent of
  crystallite /particle size each surface metal
  atom is a potential active site example
  hydrogenation, dehydrogenation
  
  
  
  
  

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Active Sites-Oxides /Sulfides.Catalysis by Ions
at surfaces

• Bronsted Lewis acids in solution
• Solid acid catalysts-Historical(acid-washed clays
  for cat cracking)
• L acidity of ionsNalt Ca 2ltY3ltTh 4. increases
  with charge/radius ratio.
• B acidity by ion substitution (Al for Si) in
  clays, zeolites, Al phosphates etc.
• Acidity measurement ( Total, L B ).

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Heterolytic adsorption on Ionicoxide surfaces

• Oxide Surface M? -O ?- - M? - O ?- - M?
• Lewis
  acid(e- acceptor) Bronsted base(H acceptor)
• H H-
• H2 M?- O ?- - M? - O ?- - M?
• H OH-
• H2O M? -O ?--M? - O ?- - M? ( B acid and B
  base)
• C2H5 H
  H OCH3
• C2H6 CH3OH M? -O ?- - M? - O ?- - M?

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Life Cycle of a Catalyst

• Catalyst Preparation
• Activation
• Surface reconstruction during catalytic run
• - Beneficial-Sulfiding of Re in PtRe
• - Harmful (carbon formation)
• Deactivation( poisons,coke, SA loss, leaching)
• Regeneration
• Catalyst Unloading

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Activity, Selectivity, Stability and Accessibility

• High activity per unit volume .
• High selectivity for desired product at adequate
  conversion level (STY for product gt 1?mol /
  ml/sec)
• High AccessibilityRole of transport rates of
  mass and heat.
• Long life time Regenerability.
• Thermal/mechanical strength in reaction
  conditions(sintering,crushing,attrition)
• Reproducible/economic/safe manufacture.

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CATALYST CHARACTERIZATION

• Bulk Physical Properties
• Bulk Chemical properties
• Surface chemical properties
• Surface Physical Properties
• Catalytic Performance

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Bulk Chemical Properties

• Elemental composition( of the final catalyst ),
  EPMA
• XRD,electron microscopy (SEM,TEM).
• Thermal Analysis(DTA/TGA).
• NMR/IR/UV-Vis/ EPR/ Mossbauer
• TPR/TPO/TPD
• EXAFS

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Surface Properties

• XPS,Auger, SIMS(bulk surface structure).
• Texture Surface area- porosity.
• Counting Active Sites
• -Selective chemisorption (H2,CO,O2, NH3,
  Pyridine,CO2)Surface reaction (N2O).
• Spectra of adsorbed species (IR/EPR/ NMR / EXAFS
  etc)

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Physical properties of formulated catalysts

• Bulk density
• Crushing strength attrition loss (comparative)
• Particle size distribution
• Porosimetry( micro(lt2 nm),macro(gt35 nm) and meso.

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Catalyst Activity Testing Definitions-
Activity

• Activity may be expressed as
• -Rate constants or TON from kinetics
• -Rates/weight
• -Rates/volume
• -Conversions at constant P,T,and SV.
• - Temp required for a given conversion at
  constant partial total pressures
• - Space velocity required for a given
  conversion at constant pressure and temp

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Catalyst Activity TestingDefinitions- Selectivity

• Selectivity concentration of product(s) among
  all the products excluding coke.
• Yield conversion X selectivity.
• Selectivities may depend on T,P,SV,diffusion,
  catalyst particle size and shape , reactor
  geometry etc.
• Always compare selectivities at constant T,P and
  most important,conversion.
• Selectivity w.r.t. each of the reactants(H2O2).

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Catalyst Testing- 1

• What is the objective ?Testing a solid for its
  catalytic properties in many reactions?screening
  for a particular reaction? Exploring
  Kinetics?Industrial development?
• Activitycomparison at non-diffusion
  non-thermodynamically limited, kinetically
  controlled conditions
• 10-20 meshdreactor gt10diacat(wall effects)
• Bed length/ dreactor gt5 to avoid channeling
• Comparison of Selectivity at similar activity

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Catalyst Testing-2

• Only at intermediate conversions and at low temp
  can the quality of the catalyst, expressed in an
  optimum of kinetically controlled conversion,be
  analyzed.At high temp or at high conversions,all
  catalysts are almost equal for either slow
  kinetic control or thermodynamically limited
  conversion.

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Start-Up Procedures Affect Catalyst Performance
Activated as per manfacturers instruction
Activated Rapidly
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Temperature dependence of catalytic activity
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Catalyst Preparation Formulation -1

• Catalyst Formulation
• - Size and shape is a compromise between the
  wish to minimize pore diffusion effects( small
  size)and pressure drop( large size)
• - Pelleting,extrusion,granulation,spray
  drying Choice depends on properties of powder,
  size/shape/density/ required strength of catalyst
  particle
• -Loading of graded sized pellets.

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Catalyst Preparation Formulation-2

• Unsupported Metals
• - very high activity(small area adequate )
• - High purity feedstock
• eg NH3 ? NO ( Pt-Rh gauze).
• CH3OH ?HCHO (Ag granules)
• - Raney Ni,Co,Cu for H2 ion (residual Al2O3
  present!).

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Catalyst Preparation Formulation- 3

• Fused catalysts.
• eg Triply promoted Fe ( Ca,K,Al as oxides)
  catalyst for NH3 synthesis.
• Fe3O4 H2(N2 H2) ? Fe(1600C)
• Melt the mixture at 1600 C,cool,crush,size.

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Catalyst Preparation Formulation- 4

• Wet methods of catalyst manufacture
• (A) Precipitation pH of precipitating medium
  critical !!
• (B)Precipitation-deposition texture of support
  important.
• Influence of Ageing,digestion filterability
• washability of salts

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The pH of precipitation affects chemical
composition, particle size and other physical
properties of Cu/ZnO/Al2O3 WGS shift catalyst
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Catalyst Preparation Formulation- 5

• Supported Metal(especially noble metals)
  Catalysts
• Used Extensively in industry
• -autoexhaust, diesel oxidation, DeNOx,
  stationary power sources
• - Hydrocracking,Naptha reforming,xylene isom,
  isomerisations, Hydrogenations, etc
• - Fuel cell catalysts
• - Major issues high cost and loss of
  activity due to sintering .

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Why the need for high dispersion of PM

• PM are expensive hence impregnation and not
  coprecipitation
• Activity depends on metal surface area (MSA)
• MSA increases with dispersion

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Metal Dispersion

• Metal Dispersion, D No of Pt surface atoms /
  No of Total Pt atoms
• D is an operational definition (defined by
  technique used)
• N total from chemical composition
• N surface is obtained by physical or chemical
  methods
• Physical methods Crystallite size from XRD,
  SEM/TEM
• Chemical methods Chemisorption of H2, CO, H2-O2
  titration
• PM distribution Profiles

a.Uniform b.Egg shell c.Egg white d.Egg yolk
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PM distribution profiles

• Optimal dispersion depends on
• reaction kinetics and mode of catalyst poisoning
• Attrition strength of catalyst
• Egg shell favors
• Reactions with positive order
• Fast reactions
• - Egg Yolk favors
• Reactions with negative order
• Pore mouth poisoning egg white or egg
  yolk
• Low attrition strength egg white or egg
  yolk

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Factors affecting dispersion of PM -1

• Concentration of PM
• Low concentration high dispersion
• Presence of competing ions in impregnating
  solution increases D.
• Citric acid in H2PtCl6 impregnation on Al2O3
  platforming)

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Factors influencing dispersion of PM -2

• 3. Functional groups on substrate surface for
  binding the PM precursor Point of zero charge
  (PZC) influences dispersion of PM
• Anions and neutral complexes disperse better on
  gamma Al2O3 at pHlt8

PZC gamma alumina8-9 SiO23
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Factors influencing dispersion of PM -3

• 4. Crystallite size of substrate
• Al2O3, CeO2, CZO, TiO2 etc
• Small crystallite sizes have large dispersion
• 5. Partially reducible oxide supports increase D
  eg Pt-CeO2
• 6. Ion exchange of PM increases D, eg Pt in
  zeolites

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Sintering of PM

• Leads to lower dispersion, MSA and activity
• Increases with PM loading
• Increases with T, TOS, H2O, O2, S, Cl
• Increases with crystallite size of support
• Increases with hydrophobicity of support (Pt-SiO2
  sinters more than Pt-Al2O3)
• Suppressed by spacers (ZrO2 in CZO)
• Suppressed by binding groups on surface (OH,
  Cl-, SO3H- etc)

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Reverse Micro Emulsion (RME) method enables use
of lower amount of Pt in DeNOx

• Nissan WO 2005/063391A1, PCT WO 2006/067912 A1
  and others
• Catalyst was first used in a Nissan engine using
  gasoline fuel for 30 hrs at 700ºC.
• After engine durability test for 50 hrs at 70ºC,
  catalyst was tested in test rig at 350ºC for
  DeNOx activity.
• Catalyst100g/l in honeycomb Pt-Co(Ce)-Al2O3

0.5 Pt is as effective as 3wt Pt
At 350ºC after endurance test at 700ºC for 30 hrs
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Some Developments in Industrial catalysis-11900-
1920s

• Industrial Process
  Catalyst
• 1900sCO 3H2 ? CH4 H2O Ni
• Vegetable Oil H2 ? butter/margarine Ni
• 1910sCoal Liquefaction
  Ni
• N2 3 H2 ? 2NH3
  Fe/K
• NH3 ?NO ?NO2 ?HNO3 Pt
• 1920s CO 2 H2 ? CH3OH (HP) (ZnCr)oxide
• Fischer-Tropsch synthesis
  Co,Fe
• SO2 ? SO3 ?H2SO4
  V2O5

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Heterogeneous Catalysis.Some Challenges Ahead

• Selective oxdn of long chain paraffins to
  terminal alcohols/ald/acids
• CH4 ?CH3OH.
• Activation of CO2 its use as raw material
• CO2 H2O/ CH3OH/C2H5OH ? C2
• Chiral catalysis with high ee.
• H2 generation from H2O without using HC .
• Photocatalysis with Sunlight.

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Industrial catalysis-21930s and 1940s

• 1930sCat Cracking(fixed,Houdry) Mont.Clay
• C2H4 ?C2H4O
  Ag
• C6H6 ? Maleic anhydride V2O5
• 1940sCat Cracking(fluid) amorph. SiAl
• alkylation (gasoline)
  HF/acid- clay
• Platforming(gasoline)
  Pt/Al2O3
• C6H6 ?C6H12
  Ni

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Industrial catalysis-3 1950s

• C2H4 ?Polyethylene(Z-N) Ti
• C2H4 ?Polyethylene(Phillips) Cr-SiO2
• Polyprop Polybutadiene(Z-N) Ti
• Steam reforming Ni-K-
  Al2O3
• HDS, HDT of naphtha (Co-Mo)/Al2O3
• C10H8 ? Phthalic anhydride (V,Mo)oxide
• C6H6 ? C6H12 (Ni)
• C6H11OH ?C6H10O (Cu)
• C7H8 H2 ?C6H6 CH4 (Ni-SiAl)

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Industrial catalysis-4 1960s

• Butene ?Maleic anhydride (V,P) oxides
• C3H6 ?acrolein (BiMo)oxides
• C3H6 ? acrylonitrile(ammox) -do-
• Bimetallic reforming PtRe/Al2O3
• Metathesis(2C3 ?C2C4) (W,Mo,Re)oxides
• Catalytic cracking
  Zeolites
• C2H4 ?vinyl acetate
  Pd/Cu
• C2H4 ? vinyl chloride
  CuCl2
• O-Xylene ?Phthalic anhydride V2O5/TiO2
• Hydrocracking
  Ni-W/Al2O3
• COH2O ?H2CO2 (HTS) Fe2O3/Cr2O3/MgO
• --do-- (LTS)
  CuO-ZnO- Al2O3

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Industrial catalysis-5 1970s

• Xylene Isom( for p-xylene) H-ZSM-5
• Methanol (low press)
  Cu-Zn/Al2O3
• Toluene to benzene and xylenes H-ZSM-5
• Catalytic dewaxing
  H-ZSM-5
• Autoexhaust catalyst Pt-Pd-Rh on
  oxide
• Hydroisomerisation
  Pt-zeolite
• SCR of NO(NH3)
  V/ Ti
• MTBE acidic ion
  exchange resin
• C7H8C9H12 ?C6H6 C8H10 Pt-Mordenite

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Industrial catalysis-6 1980s

• Ethyl benzene
  H-ZSM-5
• Methanol to gasoline
  H-ZSM-5
• Vinyl acetate
  Pd
• Oxdn of t-butanol to MMA Mo
  oxides
• Improved Coal liq NiCo sulfides
• Syngas to diesel
  Co
• HDW of kerosene/diesel.GO/VGO
  Pt/Zeolite
• MTBE cat dist ion exchange
  resin
• Cyclar
  Ga-ZSM-5
• Oxdn of methacrolein Mo-V-P
  heteropolyacid
• N-C6 to benzene
  Pt-L zeolite

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Industrial catalysis-7 1990

• DMC from acetone Cu
  chloride
• NH3 synthesis
  Ru/C
• Phenol to HQ and catechol TS-1
• Isom of butene-1(MTBE) H-Ferrierite
• Ammoximation of cyclohexanone TS-1
• Isom of oxime to caprolactam
  TS-1
• Ultra deep HDS
  Co-Mo-Al
• Olefin polym Supp. metallocene
  cats
• Ethane to acetic acid Multi component
  oxide
• Fuel cell catalysts Rh, Pt,
  ceria-zirconia
• Cr-free HT WGS catalysts Fe,Cu-
  based

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Industrial catalysis-8 2000

• Solid catalysts for biodiesel
• - solid acids, Hydroisom catalysts
• Catalysts for carbon nanotubes
• - Fe (Ni)-Mo-SiO2

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ACKNOWLEDGEMENT

• Members of the catalysis division at NCL